4x4 3:73 can i go lower

[peglegs]

i recently purchased this 87 1/2 t. 4x4 305 cu automatic when doing a rear brake job i noticed a lot of play turning the tire b/4 the opposite tire moved so that in conjunctuion with a clunk when shifting in reverse the mechanic says he sure its the spider gears? in any case they are 3:73 would i be ok with a lower gear ratio rear end. dont plan on doin to much with the 4x4 only a few muddy roads/hills or rocky logging roads occasionally. its less expensive to replace the rear than have it repaired & do a little better on gas milage thank you.

[special-K]

I gather you're talking about a numerically lower gear ratio,like 3.08s,which is higher gearing. People get confused between higher and lower. Some say lower gears and mean lower ratio (meaning the numbers are closer to 1 (3.08:1). Others say lower gears and mean low speed which is a higher ratio (4.10:1). When something's geared low it's a high ratio and when it's geared high it's a low ratio. You can see where the confusion comes from.
So,I'm thinking you want what I call taller gears,as in a gear that with roll at higher speeds and lower rpm...3.08. The answer is you's be fine. You give up some grunt but it will still move out fine. What size tires do you run?

[donut]

What transmission and tires?
OD and a low numerical ratio you can go to far in the opposite direction and hurt mileage.
And for the new guys, don't forget, in a 4x4 both axles have to match. (same ratio)

[piecesparts]

If you go to a 3.42 or a 3.08 gear ratio, you could gain some highway mileage, but it will not be greatly noticeable. You will end up putting yourself into an area low and outside the engine's power band and it will hurt your mileage. The route to the other end of the spectrum will drive your RPMs so high that you will lose mileage due to overdoing the RPMS. I found that a a 3.73 gear ratio is a great compbination for one that wants to do some rough roads and hills. I have had some experience with my truck and my son's truck on gearing and tranny combinations. We finally settled on the 3.73 gearing to get the acceleration and general road use capability.

[wilkin250r]

3.73 is my prefered ratio for almost any truck with stock tire size, best combination of trail-friendly, towing, and fuel economy.

However, let me tackle the your problem as a whole. I think it would be MUCH cheaper and easier for you to just change the spider gears, (provided that is indeed the problem). They're actually not too difficult to do yourself, the hardest part is that the pinion shaft bolt is very often broken. But aside from that obstacle, it's actually pretty straight forward.

You don't have to pull the whole carrier, so you don't have to set up your preload and backlash on the ring and pinion, it really is just a matter of pulling the cover, removing the pinion shaft bolt(like I said, it's often broken, so that can be difficult), then removing the pinion shaft . Then removing the C-clips, pulling the axles, and then removing all the spider gears. Installation is the reverse. It's honestly so simple, even a beginner can accomplish it in just a few hours.

If you had to pull the whole carrier, I'd say take it to a professional. If you had to replace the ring and pinion, I'd say take it in. But honestly, the spider gears are simple enough I'd say you can do it, there's no special clearances or difficult setup with the spider gears like there is with the ring and pinion. And they don't break often, so a junkyard set will often suffice, making them cheap as well as easy.

[Firebirdjones]

Wilkin is right, spider gears are easy to replace,,,,so long as it's an open carrier. If it's some form of posi (GM used a Gov Loc by this time frame) there are alot of things in the way and it gets more complicated.

As far as gears, being an 87 it should have a 700r4 overdrive. In my opinion the 3.73 work very nice with that trans.

I run this gear in my 72 4x4 blazer along with a 6.0/4L60E and 33" tires. It's a pretty sweet setup. I ran 3.07's in the truck before that, with everything else being the same. It worked okay but I found the truck locking and unlocking the converter when I would hit slight grades and that got annoying. Even the 6.0 LS in front didn't like it, so your 305 certainly won't either.
Really didn't help mileage either (probably because of the converter locking and unlocking) I still get 17/22 city/highway with the 3.73's,,,,same as before.

My advice, especially if you want to eventually go up in tire height in the future,,,stick with the 3.73's. Like mentioned, you would have to swap the front diff as well.

[bilfman]

Personally I would stick with the 3.73 ratio. But as to the spider gears I would toss them and put in a locker. They are very reasonably priced and will be a great upgrade for your truck and not a whole lot more work then replacing spider gears. I have put a rear locker in every truck that I have owned and would never go back to an open diff (spider gears).

[DetroitIron]

All good advice based on the little experience I've had...i have 3.42s in my K20 and the 700R4 trans makes it ok, but I bet 3.73 would be just right for me. Good luck

[355bullet]

i have 3:73's in both my 4x4's with 33's and 700r4, they run perfect.

my brother had 3.42's in his jimmy, ran weak. He up graded to 35's and 4:56's huge difference.

[peglegs]

Special k =yes numerically lower than the 3:73 perhaps the 3:42's

Donut= code MX1 on the build sheet is a turbo 350 so yes no OD, wished it was a 7004r though hope i got this one right

Pieceparts =great info makes good sense to me to now consider staying with the 3:73

Wilkin= im an old back yard mechanic & you really put my rear end repair fear at ease here if the spider gears are in fact the problem ill def do it but need to be sure can anyone help with a more definitive diagnosis. the symptoms are pounding when put in reverse & when on jacks turning one tire its perhaps 8-10" b-4 the other tire wants to move. Any other way i can check ???

Firebird =not a posi

Bilfman only lookn to repair it if a 373 came along with posi at a decent price i might this is chevy truck country here with parts & trucks for sale almost daily

Any suggestions on a more definite way to diagnose the spider gears?

[07XCSBZ71]

The Clunk could be your universal joints also, Might just be a 30 dollar fix( 15.00 each joint)

[peglegs]

Originally Posted by 07XCSBZ71

The Clunk could be your universal joints also, Might just be a 30 dollar fix( 15.00 each joint)

clunks in reverse only

[wilkin250r]

As a backyard mechanic, I'd do a $20 diagnosis. Basically, crawl under there and look at it.

My method would be to block the front tires really well, forward and backwards, and then put the tranny in neutral. This allows you to rotate the driveshaft somewhat and look for the clunk. Just as 07XCSBZ71 said, check the u-joints first.

If the U-joints aren't the problem, I'd pull the rear differential cover off, and that's where the $20 comes in, you'll need to refill it. That will tell you if it's the spiders, something wrong in the case, or I've seen a rare occasion when the pinion gear bearings went out and the clunk was caused by movement and slop between the ring and pinion gears.

Pulling off the differential cover isn't too terribly bad, you just need an oil pan to catch the differential fluid, a differential cover gasket or gasket sealer, and replacement diff. fluid when you put it back together. Also, a willingness to get dirty.

If you really want to know what you're getting into before you actually do it, there's a decent DVD called "Differential Rebuilding", the guy in the video is names Paul Zank. I ordered mine from Ebay, it will walk you through everything. Don't let the ring and pinion setup scare you, because you won't be doing that, but you'll see exactly how easy it is to pull the spider gears and replace them. The DVD will walk you through EVERYTHING, but you obviously won't be doing all of it.